1. Field of the Invention
The present invention relates to a scroll type fluid displacement apparatus, and more particularly to a mechanism for preventing the occurrence of excessive capacity and pressure in such fluid displacement apparatus.
2. Description of the Prior Art
Scroll type fluid displacement apparatuses are well known in the prior art. Generally, a scroll type fluid displacement apparatus has a first scroll member having a first spiral element and a second scroll member having a second spiral element. The first and second spiral elements are interfitted at an angular and radial offset to make a plurality of line contacts which define at least one pair of sealed off fluid pockets. The fluid pockets are moved inwardly along the spiral elements and changed in volume or displaced by relative orbital motion between the first and second scroll members. The scroll type fluid displacement apparatus includes a suction chamber formed in a housing for receiving the fluid which forms the fluid pockets, and a discharge chamber formed in the housing for discharging the displaced fluid.
There are two basic types of scroll type fluid displacement apparatuses. One basic type is a fixed system scroll type fluid displacement apparatus. In this type of scroll type fluid displacement apparatus, one of the scroll members is fixedly disposed within a housing (the "fixed scroll member") and the other scroll member is disposed for nonrotatable orbital movement relative to the fixedly disposed scroll member (the "orbiting scroll member"). The other basic type scroll type fluid displacement apparatus is a full rotational system scroll type fluid displacement apparatus. In this type of scroll type fluid displacement apparatus, both scroll members are rotated. The rotational axis of the first scroll member and the rotational axis of the second scroll member are offset by a length corresponding to the radius of the relative orbital movement of the scroll members. The scroll members rotate substantially synchronously while performing the relative orbital motion.
In conventional scroll type fluid displacement apparatuses, particularly in the conventional full rotational system scroll type fluid displacement apparatus which may be used as a compressor in an air conditioner for a vehicle, the capacity and power consumption of the compressor increases undesirably when the compressor is rotated at a high speed. As a result, the load on an engine of the vehicle increases and it becomes difficult for the air conditioner to deliver a comfortable level of air conditioning.
Moreover, in both basic types of conventional scroll type fluid displacement apparatuses, when fluid pressure increases significantly, that is, when compression of the fluid is excessive, the apparatus may be damaged. The occurrence of excessive pressure decreases the durability of the apparatus.
In fixed system scroll type fluid displacement apparatuses, mechanisms have been provided for reducing the capacity of the compressor when the compressor is rotated at a high speed. Such a mechanism is disclosed in JP-B-SHO 56-32468 and depicted in FIG. 3 of the appended drawings. In the compressor of FIG. 3, capacity reduction mechanism 301 is provided to release pressure. This mechanism, which comprises hole 302, ball 303 and spring 304, is provided at a central portion of end plate 305 of orbiting scroll member 306. Hole 302 provides fluid communication between fluid pocket 307 and suction chamber 308 when ball 303, which is biased by spring 304, is radially moved in response to centrifugal force.
In the above compressor, however, there are a number of disadvantages to the use of capacity reduction mechanism 301. First, since capacity reduction mechanism 301 is provided at the central portion of the scroll member, the high pressure of the compressed fluid cannot be reduced unless the compressed fluid reaches the central portion. If excessive pressure is generated before the compressed fluid reaches the central portion, excessive pressure still is applied to the scroll members including fixed scroll member 309. Moreover, since the reduction in capacity is performed by releasing the compressed fluid into suction chamber 308 through hole 302 after actual compression, fluid at high-temperature and high-pressure enters the suction chamber. As a result, the temperature of the compressor increases excessively and the durability of the compressor is reduced. Lastly, it is noted that the direction of the centrifugal force for moving ball 303 is different from the direction of the urging force of spring 304. Therefore, it is difficult to properly control the opening and closing of hole 302 with ball 303 as a function of rotational speed.